A mass spectrometer (MS) typically includes an ion source for producing charged species from an introduced sample, a mass analyzer for separating the charged species according to their mass-to-charge ratios (m/z ratios, or simply “masses”), and an ion detector for counting the separated species to provide signals from which mass spectra may be produced. The sample may be introduced into the ion source by various techniques. In one example, a gas chromatograph (GC) is interfaced with the MS such that the sample output from the GC column—containing chromatographically separated sample components—serves as the sample input into the ion source. The latter system is often termed a GC/MS system.
As an MS continues to be operated over time, invariably some alteration or degradation in the performance of the MS occurs due to the samples, their matrix (e.g., heavy hydrocarbons in petroleum samples, triglycerides in fat samples) and solvents, stationary phase bleed from the GC column, or other recalcitrant substances, all of which may accumulate over time. Even at the initial operation of the MS, the MS may not be stabilized or “conditioned” to provide adequate or uniform performance. In the case of gas chromatography where an electron impact (EI) or chemical ionization (CI) source is typically utilized in the MS, the ion source can be rapidly fouled by the introduced sample components, which results in degraded performance as seen in the analyte signal or spectral characteristics. Another problem, especially with high-boiling analytes, is that peak tailing can increase with continued use in addition to reduced signal response. The degraded performance may be manifested in many ways but typically the metrics are reduced analyte signal response and high system background noise, the latter being particularly troublesome for analyte detection and identification.
These problems require that the MS be cleaned periodically. Generally, the higher the rate of contaminant deposition, the more often the MS must be cleaned. The common, conventional cleaning solution has been to vent the MS system, remove the critically affected components (e.g., ion source, ion optics, pre-filter, etc.), treat the removed components to mechanical and/or chemical cleaning followed by other processes (e.g., muffle or vacuum furnace baking), and then re-install the components in the MS system. Such conventional ex situ cleaning procedures can be quite complex and lengthy procedures, involving potentially toxic solvents, expensive equipment, and the time and care of skilled technicians. Moreover, the cleaning process only temporarily solves the problem. After performing an iteration of cleaning and resuming the analytical operation of the MS, the performance of the MS will start to degrade again, eventually requiring another iteration of cleaning. In addition, the conventional cleaning process may fail due to mechanical issues associated with the reinstallation of components, or because some step in the procedure was compromised (e.g., a cleaning solvent was contaminated). Such failures may not be discovered until the MS is reassembled, under vacuum, and at operating conditions. Also, the process of venting entrains certain background species, the most abundant of which is water, which results in additional time being required to eliminate these substances. Water as a contaminant can cause a rapid reduction in MS signal response.
Helium is the most commonly employed carrier gas in GC/MS due to helium's inertness, low mass, high ionization potential, and desirable chromatographic properties. Moreover, spectral reference libraries such as NIST 08 (National Institute of Standards and Technology) are recorded using helium as the carrier gas. Helium alone, however, does not possess any inherent cleaning or conditioning properties, and hence its use as a carrier gas cannot ameliorate the need for frequently carrying out the above-described cleaning procedures. It would thus be desirable to provide a solution to this problem that prevents the response loss and tailing that occurs when using helium as a carrier gas, and/or reduces or eliminates the need to clean the MS system by the above-described conventional techniques while still retaining the benefits of using helium as a carrier gas.
Hydrogen has also been employed as a carrier gas, but much more rarely than helium and other carrier gases. A number of significant disadvantages attend the use of hydrogen as a carrier gas. Hydrogen is highly combustible. The choice of column dimensions is severely limited with hydrogen due to its low viscosity. Much smaller columns are required to maintain a positive inlet pressure when the column outlet is an MS. The signal-to-noise ratio of a mass spectrum or chromatogram when using hydrogen as the carrier gas is much lower, resulting in detection limits that are five to ten times worse than when using helium as the carrier gas. The use of hydrogen can lead to degradation reactions of analytes in the ion source, resulting in a peak tail having a different composition than the apex of the associated peak. This comprises the spectral fidelity, which is an important factor in analyte identification when employing spectral library searches. Also, the presence of hydrogen in the sample inlet and the column can result in chemical reactions with analytes that change their structure.
In view of the foregoing, there is an ongoing need in mass spectrometry, including gas chromatography/mass spectrometry, for methods and apparatus for conditioning an MS system. There is also a need for methods and apparatus for in situ conditioning that is carried out at the MS system, whereby the need for conventional ex situ cleaning is eliminated or at least significantly reduced. There is also a need for methods and apparatus that make effective use of hydrogen and/or other gases in MS systems as an alternative to, or in conjunction with, more common gases such as helium.